Two new options:

- server option "exit-unattached" makes the server exit when no clients
  are attached, even if sessions are present;

- session option "destroy-unattached" destroys a session once no clients
  are attached to it.

These are useful for preventing tmux remaining in the background where
it is undesirable and when using tmux as a login shell to keep a limit
on new sessions.

cmd-switch-client.c

@@ -152,6 +152,7 @@ cmd_switch_client_exec(struct cmd *self, struct cmd_ctx *ctx)
 	c->session = s;
 
 	recalculate_sizes();
+	server_check_unattached();
 	server_redraw_client(c);
 
 	return (0);